Offset acetabular cups: A solution to wear?

Change of tribological properties has been the main goal in the effort to augment the longevity of total hip arthroplasty, while the reduction of forces acting across the artificial joint space has received little attention. Spurred by recent reports of wear behaviour and good clinical results of the offset low friction arthroplasty cups, the authors, using the simplified free body technique, estimated the resultant hip joint reaction force and the angle of its application on 100 individual anteroposterior pelvic radiographs, by templating on the same hip a conventional concentric cup and an offset eccentric one. The results showed a highly significant reduction (p<0.0001) of the resultant hip joint reaction force in the offset group by 28.8% or 1.02 body weight and of the angle of the hip joint reaction force by 2.8 degrees. The authors believe that the reduction of the resultant hip joint reaction force in the offset cup group is the result of lowering and medialising the centre of rotation of the hip, a previously reported fact on non-clinically applicable conditions. They are also tempted to propose the generalized use of the offset type acetabular cup, since adverse events are not anticipated. (Hip International 2005; 15: 149-54).